12.2.2 A Simple Lambda Expression Example

Consider the following example:

(lambda (a b c) (+ a b c))

We can call this function by passing it to funcall, like this:

(funcall (lambda (a b c) (+ a b c))
1 2 3)

This call evaluates the body of the lambda expression with the variable
a bound to 1, b bound to 2, and c bound to 3.
Evaluation of the body adds these three numbers, producing the result 6;
therefore, this call to the function returns the value 6.

Note that the arguments can be the results of other function calls, as in
this example:

(funcall (lambda (a b c) (+ a b c))
1 (* 2 3) (- 5 4))

This evaluates the arguments 1, (* 2 3), and (- 5
4) from left to right. Then it applies the lambda expression to the
argument values 1, 6 and 1 to produce the value 8.

As these examples show, you can use a form with a lambda expression
as its car to make local variables and give them values. In the
old days of Lisp, this technique was the only way to bind and
initialize local variables. But nowadays, it is clearer to use the
special form let for this purpose (see Local Variables).
Lambda expressions are mainly used as anonymous functions for passing
as arguments to other functions (see Anonymous Functions), or
stored as symbol function definitions to produce named functions
(see Function Names).